Real-world perspective display for use with an independent aircraft landing monitor system

ABSTRACT

A RADAR DISPLAY FOR AN INDEPENDENT AIRCRAFT LANDING MONITOR WHEREIN A RADAR SYSTEM ONBOARD AN AIRCRAFT DETECTS AN AIRPORT RUNWAY DURING GLIDESLOPE APPROACH. THE DISPLAY INCLUDES A RADAR DISPLAY TUBE HAVING HORIZONTAL AND VERTICAL SWEEP CIRCUITS FOR CONTROLLING THE VISUAL DISPLAY OF THE TUBE. CIRCUITRY OPERATES THE HORIZONTAL SWEEP CIRCUITS OF THE DISPLAY TUBE IN SYNCHRONISM WITH THE AZIMUTH SCAN OF THE RADAR SYSTEM. CIRCUITS ARE RESPONSIVE TO THE AIRCRAFT ALTITUDE FOR GENERATING A DRIVING OUTPUT SIGNAL WHICH VARIES NON-LINEARLY IN AMPLITUDE WITH RESPECT TO THE RATIO OF AIRCRAFT ALTITUDE AND RADAR RANGE. CIRCUITRY IS RESPONSIVE TO THE DRIVING OUTPUT SIGNAL FOR CONTROL OF THE ELEVATION ANGLE DISPLAYED BY THE VERTICAL SWEEP CIRCUITS OF KTHE RADAR DISPLAY TUBE TO THEREBY GENERATE INDICATIONS OF THE APPROACHING AIRPORT RUNWAY IN REAL-WORLD PERSPECTIVE. PROVISION IS ALSO MADE TO SELECTIVELY DISPLAY CONVENTIONAL B SWEEP OR PPI DISPLAYS OF THE APPROACHING AIRPORT RUNWAY, AND RANGE MARKER SIGNALS ARE DISPLAYED UPON THE RADAR DISPLAY TUBE DURING GLIDESLOPE APPROACH.

Feb. 13, 1973 J. s. MASON 3,716,366

REAL'WORLD PERSPECTIVE DISPLAY FOR USE WITH AN INDEPENDENT AIRCRAFTLANDING MONITOR SYSTEM FIG.I

Feb. 13, 1973 J. 5. MASON 3,716,366 REAL-WORLD PERSPECTIVE DISPLAY FORUSE WITH AN INDEPENDENT AIRCRAFT LANDING MONITOR SYSTEM Filed July 15,1970 125 Sheets-Sheet FIG. 5

Feb. 13, 1973 J. 5. ASON REAL-WORLD PERSPECTI DISPLAY FOR USE WITINDEPENDENT AIRCRAFT LANDING MONITOR SYST 70 i5 Sheets-Sheet 4 1| WW WWWHMHwHHUI I H 1.,1|.1|1%1,||il' 1|.IL

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Feb. 13, 1973 J. 5. MASON 3,716,866

REAL'WORLD PERSPECTIVE DISPLAY FOR U515 WITH AN INDEPENDENT AIRCRAFTLANDING MONITOR SYSTEM Filed July 15, 1970 15 Sheets-Sheet 6 5x5 356 E5293 wow 2w ovw x motzoz J E on wvw 51v zutzm BEER EN EN com mmm/ 2 6 Tuwalw Feb. 13, 1973 J. s. MASON 3,716,866

HEAL-WORLD PERSPECTIVE DISPLAY FOR USE WITH AN INDEPENDENT AIRCRAFTLANDING MONITOR SYSTEM Filed July 15, 1970 L3 Sheets-Sheet 9 FIG, I

HEADLINE Feb. 13, 1973 J. 5. MASON 3,716,866

REAL-WORLD PERSPECTIVE DISPLAY FOR USE Wl'IH AN INDEPENDENT AIRCRAFTLANDING MONITOR SYSTEM Filed July 15, 1970 13 Sheets-Sheet 10 L .m.-2-.i:. TOUCHDOWN O TOUCH- T DOWN BETA -|5 T -|5O ALPHA +150 BETA (a) a5I5.0 ALPHA- +\5.o

b) O 5/6 5/3 /5I4 TOUCHDOWN l TOUCH T 12 1mm BETA 522 A5 T 150 ALPHA+|5.0 BETA (0) W 2 9 2m L1.) J LLJ 8 FIG. I? Q 4 2 ,fl o

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J. S. MASON Feb. 13, 1973 N A M E T MS Y S m HUT RI N W O M Y. m m 1 D mD L E T HF A CR C R 1 M A P W MN 0 m m MN U1 l5 Sheets-Sheet 1 FiledJuly 15, 1970 ON 6E omnzrm mkm Feb. 1-3, 1973 J. 5. MASON 3,716,366

REALWORLD PERSPECTIVE DISPLAY FOR USE WITH AN INDEPENDENT AIRCRAFTLANDING MONITOR SYSTEM Filed July 15, 1970 15 Sheets-Sheet 12 SWEEPWIDTH ONE ,SHOT MV FIG. 2!

Feb. 13, 1973 J. s. MASON REAL'WORLD PERSPECTIVE DISPLAY FOR USE WITH ANINDEPENDENT AIRCRAFT LANDING MONITOR SYSTEM Filed July 15, 1970 isSheets-Sheet 15 SHAPED VOLTAGE 5|O Winona Pg United States PatentREAL-WORLD PERSPECTIVE DISPLAY FOR USE WITH AN INDEPENDENT AIRCRAFTLANDING MONITQR SYSTEM James S. Mason, Garland, Tex., assignor to TexasInstruments Incorporated, Dallas, Tex. Filied July 15, 1970, Ser. No.55,166 Int. Cl. 601s 7/24 US. Cl. 343- LS 25 Claims ABSTRACT OF THEDISCLOSURE A radar display for an independent aircraft landing monitorwherein a radar system onboard an aircraft detects an airport runwayduring glideslope approach. The display includes a radar display tubehaving horizontal and vertical sweep circuits for controlling the visualdisplay of the tube. Circuitry operates the horizontal sweep circuits ofthe display tube in synchronism with the azimuth scan of the radarsystem. Circuits are responsive to the aircraft altitude for generatinga driving output signal which varies non-linearly in amplitude withrespect to the ratio of aircraft altitude and radar range. Circuitry isresponsive to the driving output signal for control of the elevationangle displayed by the vertical sweep circuits of the radar display tubeto thereby generate indications of the approaching airport runway inreal-world perspective. Provision is also made to selectively displayconventional B sweep or PPI displays of the approaching airport runway,and range marker signals are displayed upon the radar display tubeduring glideslope approach.

This invention relates to radar systems, and more particularly to aradar display for use with an independent aircraft landing monitor whichdetects an airport runway during glideslope approach and landing.

A number of aircraft guidance systems have previously been developed forassisting the landing of aircraft. However, the advent of large, highspeed jet passenger aircraft and the development of automatic landingsystems, such as flight director systems, have resulted in the need foran onboard aircraft landing monitor which is independent of ground-basedelectronic guidance equipment to enable the pilot to progressivelymonitor the final aircraft approach, touchdown and rollout.Specifically, the need has arisen for an onboard independent landingmonitor which provides the pilot with positive assurance that theaircraft loc'alizer approach is valid, that the aircrafts true positionrelative to the runway center line and threshold is satisfactory andthat the airport runway is clear of obstructions. Such an independentlanding monitor is particularly desirable during blind or low visibilityaircaft takeoifs or landings, and during landings at airports which areunequipped or underequipped with navigational aids.

Radar systems have previously been utilized onboard aircraft for suchuses as terrain and aircraft avoidance. However, previously developedaircraft radar systems have not been generally useful as independentlanding monitors, due to antenna resolution deficiencies and because theradarscope display of such prior systems provides a distorted view ofthe runway shape. The resulting inaccurate runway display of such priorradar systems has made it extremely difficult for a pilot to makemeaningful landing decisions such as identifying runway and taxiways,and accurately determining the aircraft angle to the runway center line.For instance, a conventional planned position indicator (PPI) radardisplay provides accurate range and angle presentation of an approachingrunway, but this display presents a birds eye view of ice the runwaywhich gives an aircraft pilot a false sense of altitude, no sense ofurgency for landing and makes for a difficult transition to a visualinspection of the approaching runway. Additionally, the conventionalB-scope "and delayed B-scope presentation, wherein range is plottedagainst an independent variable scan angle, provides adequateidentification of runway patterns at long range Where the angulardistortion is minimal, but provides extreme distortion of the runway atthe minimum decision altitude during an aircraft landing, therebypreventing the pilot from accurately identifying the runway and taxiwaysand making it impossible to accurately determine the aircraft angle tothe runway center line.

In accordance with the present invention, a short range, high resolutionmapping radar system is located onboard an aircraft and is independentof ground-based electronic equipment to monitor runway alignment and thelike during approach, touchdown and rollout phases of aircraft landing.The present system utilizes a high resolution antenna system incombination with a visual radar display which presents a real-worlddisplay of the approaching runway to the pilot on a one-to-onecorrespondence to real-world perspective. The pilot may then accuratelyidentify the runway threshold, accurately determine the angle to centerof the runway, measure lateral offset and make a smooth transition tovisual runway information.

In accordance with the present invention, a visual display for a radarsystem includes a display surface, along with circuitry responsive to aradar system for sweeping across the display surface to generate avisual radar indication thereon. Circuitry nonlinearly controls thesweeping circuitry to provide the visual radar indication withreal-world linear perspective.

In accordance with a more specific aspect of the inven tion, a sweepgenerator is utilized to control a radarscope in an aircraft landingmonitor radar system. The sweep generator includes circuitry forcontrolling the horizontal sweep of the radarscope in response to theazimuth sweep of the radar antenna. Circuitry also controls the verticalsweep position of the radarscope in response to the aircraft altitudeand the radar target range in order to provide a display havingreal-world perspective.

In accordance with another aspect of the invention, a radar displaysystem is utilized in an aircraft landing monitor wherein radar systemtransmits and receives radar signals for detection of an airport runwayduring glideslope approach. The display system includes a radar displaytube having horizontal and vertical sweep control circuits. Circuitryoperates the horizontal sweep control circuits in accordance with theazimuth sweep of the radar system. Circuitry also operates the verticalsweep control circuits in response to the ratio of the aircraft altitudeand the instantaneous radar range to thereby display the approachingairport runway in real-world perspective.

In accordance with a more specific aspect of the invention, a radardisplay is provided for an independent aircraft landing monitor whereina radar system aboard the aircraft detects an airport runway duringglideslope approach. The display includes a radar tube having horizontaland vertical sweep circuits. Circuitry is provided to operate thehorizontal sweep in synchronism with the azimuth scan of the radarsystem. A plurality of waveshaping circuits each generate a shapedvoltage output varying in amplitude with respect to aircraft altitude. Aplurality of charging networks charge up to the voltage presented by itsrespective shaping circuit. Each of the voltage charging circuits have adifferent time constant. Circuitry is provided to sum the voltagesgenerated by the charging networks to generate an output signal forcontrol of the vertical sweep circuit of the radar display tube, whereinthe approaching airport runway is displayed in real-world perspective.

For a more complete understanding of the invention and for furtherobjects and advantages thereof, reference may now be made to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a somewhat diagrammatic illustration of the antenna radiationpattern of the invention in the elevation plane during glideslopeapproach;

FIG. 2 is a somewhat diagrammatic illustration of the azimuth sweep ofthe antenna radiation pattern of the present system;

FIG. 3 is a somewhat diagrammatic illustration of the installation ofthe present landing monitor radar system in the nose of an aircraft;

FIG. 4 is a block diagram of the present landing monitor system;

FIG. 5 is a central sectional view taken of the antenna of the presentsystem;

FIG. 6 is a top view, partially broken away, of the antenna shown inFIG. 5;

FIG. 7 is a front view, partially broken away, of the antenna shown inFIG. 5;

FIG. 8 is a perspective view of a portion of the slotted waveguideassembly utilized in the antenna shown in FIG. 5;

FIG. 9 is a block diagram of the antenna sweep control circuitry of theinvention;

FIG. 10 is a block diagram of the radar receiver utilized in the presentsystem;

FIG. 11 is a block diagram of the radar transmitter utilized in thepresent system;

FIG. 12 is a block diagram of the sweep generation circuitry utilized tocontrol the radar display of the present invention;

FIGS. 13 and 14 are diagrammatic illustrations of the relationshipsbetween the range and altitude of the aircraft in the display of therunway during glideslope approach;

FIGS. 15 and 16 are diagrams illustrating aspects of the theory ofoperation of the present radarscope display;

FIG. 17 is a graph illustrating the relationship between elevationangle, range and altitude according to the present display system;

FIG. 18 is a graphical representation of the nonlinear voltages utilizedto control the vertical sweep of the present radarscope display;

FIGS. 19ad are somewhat idealized illustrations of the presentradarscope display during approach to an airport runway;

FIG. 20 is a detailed schematic of the nonlinear voltage generators inthe sweep generator circuit of the invention;

FIG. 21 is a detailed schematic of the delay circuitry for generation ofthe unblank signal for use in the sweep generator of the invention; and

FIG. 22 is the detailed schematic of the storage, clamping and summingcircuits utilized in the sweep generator circuit of the invention.

Referring to the figures, FIGS. 1 and 2 somewhat diagrammaticallyillustrate the basic operation of the present landing monitor system. Anaircraft 10 is illustrated in a landing attitude during a glideslopeapproach toward an airport runway 12. As shown in FIG. 1, a radar beamis transmitted from the nose of the aircraft 10 with an elevationalantenna beam width 14 of about 17. As shown in FIG. 2, the azimuthantenna beam width 16 is approximately 0.4" and is continuously sweptover an azimuth sweep angle 18 of approximately The swept antenna beamconfiguration provided by the system enables high resolution radarmapping of the approaching runway 12 during landing of the aircraft 10.Radar reflectron signals from the runway 12 and the grass and terrainsurrounding the runway are received by radar receiving circuitry in thenose of the aircraft 10 and the 4 runway 12 is displayed in real-worldperspective to the pilot.

In the preferred embodiment of the invention, a maximum range of aboutfive miles, with a range of about two miles for initial runwayacquisition, is afforded to the landing monitor radar system. Aircraft10, landing with typical glideslope angles of from 2.5 to 3.0, Will thusbe about 2600 feet from the end of the runway 12 at an altitude of 200feet and about 1200 feet from the runway 12 at a decision altitude offeet.

FIG. 3 is a cutout view of the nose portion of a conventional aircraftto illustrate the basic components of the present landing monitorsystem. A mechanically swept antenna 20 is located in the nose of theaircraft and comprises an elongated edge-slotted waveguide array 22which is receiprocated about a vertical axis 24. As may be seen fromFIG. 2, the antenna 20 is periodically swept 15 on either side of thelongitudinal axis of the aircraft 10 to provide the 30 sweep angle 18.In the preferred embodiment, antenna 20 is swept at a rate of 2.5 cyclesper second.

Pulsed radar signals are transmitted via the antenna 20 through suitablewaveguide connections 26 which extend from a transmitter-receiverhousing 28. The return radar signals are received by receiver circuitrywithin the housing 28. Sweep generator circuitry is contained within ahousing 30 and provides electrical signals via leads 32 to a displaymonitor radarscope 34 mounted in the instrument panel of the aircraft.Display monitor 34 is preferably of the direct view storage tube typeand provides a display surface 36 wherein the pilot may receive areal-world perspective indication of the upcoming runway 12. Displaymonitor 34 includes various adjustment and select knobs to providealternative conventional B sweep and PPI display sweep modes if desired.Power supply circuitry for the system is contained within a housing 38.

The present system provides a practical independent landing monitorsystem for use on large passenger capability jumbo jet aircraft. Thepresent high resolution radar system permits visual assessment to thepilot of the aircrafts alignment with the runway center line during thefinal phase of approach, even in the event of inclement weather such asrain, snow or fog which provides zero-zero visibility.

FIG. 4 is a block diagram of the basic independent landing monitorsystem. The antenna 20 is oscillated about a rotary joint 40 by a torquemotor 42. A synchro or control transformer 44 senses the instantaneousposition of the antenna 20 and supplies a signal via a lead 46 to anantenna position demodulator circuit 48. The demodulator 48 generates aslowly varying DC voltage which is supplied to an output control circuit50 which serves to operate upon the duty cycle of voltage pulsessupplied to the torque motor 42 in order to maintain the desiredoscillation of the antenna 20. For instance, if the oscillation of theantenna 20 begins to decrease in magnitude, the output control circuit50 drives the torque motor 42 to increase the angle of antennaoscillation.

The antenna 20 is connected through a circulator 52 to a radartransmitter 54 and a radar receiver 56. Circulator 52 operates as aduplexing circuit in order to couple the antenna 20 with either thetransmitter 54 or the receiver 56. A synchronizer circuit 58 isconnected for control of the transmitter 54, and comprises a clockcircuit (not shown) containing an RC oscillator which establishes thePRF for the system. A PMT monostable multivibrator circuit (not shown)is triggered by the clock to set the pulse of the premaster trigger(PMT) of the system. The PMT is fed through buffer circuits to variousportions of the system, as will be described later in greater detail.

The PMT is fed from the transmitter 54 to the sweep generator unit andspecifically to real-world sweep circuits 60 and PPI and B sweepcircuits 62. An altitude input signal representative of the altitude ofthe aircraft is also fed into the real-world sweep circuit, and outputsare generated for application to sweep selection circuits 64. The sweepselection circuits 64 are controlled by mode switches in a control box66. When the real-world mode is selected, the real-world sweep circuits60 drive vertical and horizontal deflection amplifiers 68 to provide areal-world perspective display on the direct view storage tube (DVST)70. When the PPI mode is selected at the control box 66, the PPI sweepcircuits 62 drive the vertical and horizontal deflection amplifiers 68to provide a plan position indicator display in the well known manner atthe DVST 70. When the B sweep selection is made at control box 66, the Bsweep circuit 62 operates to drive the vertical and horizontaldeflection amplifiers 68 to provide a conventional B sweep indication atthe DVST 70. Power is provided to the various portions of' the systemfrom suitable power supplies 72.

The receiver 56 is of the quasilog type and generates signals via leads74 which are applied to video amplifiers 76. An unblank and intensitycompensation circuit 78 supplies an unblank signal to the amplifier 76.Amplifier 76 supplies the amplified video and unblanking signals to theWrite gun supply 80 to control the intensity of the electron writingbeam of the DVST '70. The intensity control 82, mounted on the frontpanel of the display, controls the gain of the unblanking amplifier 76,which in turn controls the brightness of the video information writtenon the DVST. The memory control 82, also mounted on the front panel ofthe display, is used to control the persistence (or storage time) of thedisplay. The memory control operates upon the erase generator circuitry86 and causes it to supply a variable duty cycle pulse train to thebacking electrode of the DVST 70. This in turn controls the amount ofinformation erased from the storage surface of the DV'ST. The erasegenerator 86 also supplies a pulse, coincident with the erase pulse, toa dunk tube (not shown) which drops the view screen supply 88 voltage toa very low level for the duration of the erase pulse. This eliminateslight bursts that would otherwise occur for the erase pulse duration.The flood gun supply 84 provides the necessary voltages to the DVST suchthat flooding beam is properly collimated. The receiver gain controlsupplies a signal to the receiver 56 to vary the amplitude of the radarvideo which is supplied to the display,

The DVST 70 is of conventional design, and a suitable tube for use withthe invention has a writing speed of 450,000 inches per microsecond andis manufactured and sold by Westinghouse Corporation. Basically, theflood gun of the DVST illuminates a grid which is selectively charged bythe write gun of the tube. The tube thus has persistence which isdetermined by the duty cycle of the erase generator which is pulsed toerase the image on the DVST. The unblanking circuits of the systeminhibit the display during the retrace time of the sweeps.

The unblanking circuits enable the display to write video and symbologyon the DVST during the active portion of the sweeps. The intensitycompensation circuits 78 provide a compensation signal superimposed onthe unblanking pedestal pulse to correct for the differences in sweepspeed (inches/,usec.) and the density of radar returns at difi'erentareas on the display screen. The use of the DVST 70 is advantageous inthe system as the tube supplies an integrating quality. Therefore, weakpulses receive by the system tend to be integrated due to the fact thata large number of hits (radar returns) strike the same resolution cellon the screen of the DVST.

A range tracker system 90 receives the PMT from. synchronizer 58, theradar video from receiver 56, and the antenna position signal fromdemodulator 48. The range tracker system 90 generates indications of therange of the aircraft to runway touchdown. In the preferred embodiment,a plurality of passive reflectors are arranged adjacent the airportrunway in a predetermined pseudorandom coded configuration, and thetracker system detects radar reflections from the reflectors toaccurately determine the aircraft range to the reflectors. This rangetracker system 90 is described in detail in the copending patentapplication Ser. No. 55,165, filed July 15, 1970, and entitled RangeTracking System for Use in an Independent Aircraft Landing Monitor.

The range information generated by tracker 90 is fed to the amplifiers76 for display to the pilot on the DVST 70. The range information isalso fed to a glideslope computer 92 which detects the relativeamplitudes of radar signals reflected from predetermined reflectorsadjacent the runway to generate indications of the position of theaircraft relative to the runway glideslope. The indications are fed toamplifiers 76. For a more detailed description of the glideslopecomputer, reference is made to the copending patent applicatiofiSerJNo.55,164, filed July 15,

1970, and entitled Glideslope Position Detection System for Use With anIndependent Aircraft Landing Monitor.

ANTENNA SYSTEM FIGS. 5-7 illustrate the antenna assembly 20. A generallycylindrical member is adapted to be rigidly attached to a radornebulkhead within the nose of an aircraft. A housing 102 is connected tothe cylindrical member 100 and supports the oscillating antennaassembly. A lower housing 104 contains a rotary joint 106 which enablesrotation between a fixed waveguide section 108 and a waveguide section110 which is attached to the oscillating antenna. The waveguide assembly108 is connected to the transmitter and receiving circuitry of thesystem. The entire waveguide assembly of the system is normallypressurized with Freon-116. A control transformer 111 is connected to atorque motor including a torque rotor 112 and a torque motor stator andhousing 114. The control transformer 111 senses the position of theantenna and supplies signals to the antenna control loop previouslydescribed in order to provide control signals to the torque motor tomaintain the desired oscillation of the antenna.

The output shaft 16 of the torque motor is mounted in bearings 118 andis fixedly attached to an antenna housing 120 for oscillation thereof. Ashaft 122 is fixed at the upper end of the housing 124 which is boltedonto housing 102. An outer housing 126 is connected to the oscillatableantenna and is mounted in bearings 128 and 130 for relative rotationwith respect to the fixed housing of the antenna system. The lower endof the shaft 122 is connected to a generally U-shaped member 132 whichreceives a center portion of an elongated flexible metal spring 134. Abolt 136 enables the U-shaped member 132 to be tightly clamped about thecenter of the spring 134. As best shown in FIG. 6, the spring 134extends along the length of the antenna housing 120. The ends of thespring 134 are free, but are slidably received between rollers 140 atone end, and rollers 142 at the other end thereof. As the antennahousing is oscillated, the spring 134 flexes and rotates relative to therollers 140 and 142. The spring 134 stores energy upon rotation of theantenna housing and tends to return the housing back to its previousposition.

A real housing portion 146 is attached to housing 120 to provide anaerodynamically streamlined configuration to the antenna. A forwardlyfacing radorne 148 is also attached to housing 120. Preferably, radorne148 is constructed from a material such as Fiberglas to enabletransmission of radar signals therethrough.

An antenna reflector horn 150 is disposed along the front length of theantenna and includes a waveguide 152 along the length thereof. Waveguide152 is connected to Waveguide 110. A quarter-wave grid type polarizer154 is mounted in the reflector 150 and is preferably comprised of afoam type dielectric material. A backing channel member 156 is connectedto the reflector 150.

As shown in FIG. 6, in operation the antenna is swept about the verticalaxis extending through shaft 122 about a 30 angle, or on either side ofthe head-on position of the antenna. The spring 134 assists inmaintaining oscillation of the antenna and the error signals fed fromthe control transformer control circuit back to the torque motor assistsin maintaining the oscillation of the antenna at the predeterminedangle.

In the preferred embodiment of the antenna, the antenna is approximately60 inches long and 7 inches wide. The present antenna is preferablyoperated in the Ka band of 33 to 38 gHz. With a 2.5 db antenna loss, theantenna gain for the preferred system is approximately 33.5 db.

The present antenna is of an edge-slotted waveguide array type. Moreparticularly, reference is made to FIG. 8 which illustrates a section ofthe edge-slotted waveguide 152 utilized in the present antenna. Aplurality of slots 160 slope in a first direction, While alternate slots162 out through the front face of the waveguide slant in an oppositedirection. The slots are regularly cut in the front narrow wall of thewaveguide 152 and are designed to resonate at a frequency which is twicethe waveguide wave length spacing. Thus, at resonance all of the slotsradiate in phase and the beam is oriented normal to the array length. Atresonance the slots 160 and 162 become a pure conductance and add, sincethe slots are in shunt in the transmission line circuit. At the resonantfrequency, the load is thus pure resistive and is equal to the sum ofthe slot conductances.

To properly illuminate the array, power is extracted from the travelingwave as each slot is illuminated. Because of this, the slot conductanceis increased with distance from the feed point of the waveguide, thusresulting in a total conductance much greater than unity andconsequently resulting in a large resistive mismatch at resonance. Forthis reason, the present array is operated slightly above the resonantfrequency. The beam is thus pointed approximately one to two beam widthsaway from broadside of the antenna. Under this condition, a VSWR of lessthan 1.2:1 is insured. The present antenna of the preferred embodimentwill operate at 178 wavelengths at the Ka-band. For additionaldescription of the theory of such edge-slotted waveguide arrays,reference is made to Antenna Engineering Handbook, by H. Jasik, 1961,Chapter 9, McGraw-Hill.

The quarter wave plate polarizer member 154 converts the horizontalpolarization of the slotted array feed to circular polarization. Thepolarizer 154 is a quarter wave device comprising a plurality ofregularly spaced narrow metallic vanes supported in the low-lossdielectric Eccofoam material. The orientation of the metal vanes are atan angle of 45 with respect to the antenna polarization to therebyconvert the linear polarization to circular polarization. An integratedcancellation ratio (ICR) of 17 db is readily achievable utilizing thiscircular polarization technique.

Although not shown, in the preferred embodiment of the antenna, thewaveguide 152 is routed in a second path back along the length of theantenna to maintain symmetrical loading thereof. The waveguide and thereflector antenna horn are generally constructed from aluminum andconnected in a rigid configuration. The torque motor chosen for use withthe invention in the preferred embodiment is a brushless DC torque motorwith a permanent magnet rotor and a toroid coil wound stator. The motoris rated :25" angular motion, continuous at eight watts and a20-ounce-inch peak torque. A high pressure reserve bottle of Freon-116is mounted near the far end of the waveguide and feeds the waveguidewith pressurized Freon-116 through a preset pressure reducer. A pressuregauge fitting on the bottle allows checking of the reserve supply and asimilar attachment combined with the lead valve is provided at the endof the waveguide adjacent to the load to allow verification of pressurein the waveguide and to enable bleeding of the system after maintenance.

FIG. 9 illustrates in block detail the circuit for control of theoscillatory motion of the antenna 42. The synchro 44- comprises a lineartransformer which generates a modulated 2000 cycles per second signal toan output demodulator circuit 168. A reference voltage, which preferablyis the 2000 cycle per second aircraft supply voltage, is fed to thesynchro 44 and to a reference demodulator and offset sum circuit 170.Circuit compensates the aircraft supply voltage for changes in voltageamplitude and supplies a constant voltage amplitude to a multiplier 172.Circuit 170 thus prevents detection of changes in the aircraft supplyvoltage amplitude as changes in the antenna position.

The output of the output demodulator 168 is a slowly varying DC signalat the antenna scan rate, which in the preferred embodiment is 2 /2cycles per second. The output of the "demodulator 168 is thus responsiveto the instantaneous position of the antenna 20. The output of themultiplier 172 generates a signal responsive to the antenna position tothe 90 phase shifter 174 and to the absolute value amplifier 176. The 90phase shifter 174 shifts the sinusoidal output from the multiplier 172,such that zero crossings thereof are representative of the occurrence ofpeaks of the waveforms. A positive zero crossing detector 178 generatesan electrical indication of the positive amplitude peaks of the antennaposition signal, while a negative zero crossing detector 180 providesindications of the negative voltage peaks of the amplitude positionsignal.

The output of detector 178 is fed to an input of an OR logic 182 andalso to the trigger input of a voltage variable pulse width monostablemultivibrator 184. The output of the detector 180 is also fed throughthe OR logic 182 and also to the trigger input of a voltage variablepulse width monostable multivibrator 186. The multivibrator 184 istriggered upon the occurrence of each positive peak of amplitudeposition signal, while the multivibrator 186 is triggered only upon theoccurrence of negative peaks of the antenna position signal.

The output of the OR logic 182 is fed to a sample and hold amplifier188. The absolute value amplifier 176 rectifies the position signal fromthe multiplier 172 and gives a positive voltage output indicative ofboth the left and right scan peak positions of the antenna. Thisinformation is fed to the sample and hold amplifier 188, which generatesa positive voltage representative of the peak left and right position ofthe antenna only upon the occurrence of a position peak as detected bythe OR logic 182. This voltage is applied to the multivibrators 184 and186. As the multivibrators 184 and 186 are alternatively triggered bydetectors 178 and 180, a monostable multivibrator output pulse isalternatively generated by the multivibrators which is representative ofalternating left and right antenna position peaks.

The pulse width of the multivibrator outputs is dependent upon thedetected position of the antenna. The pulse outputs from themultivibrators are fed to a driver matrix 188, which may comprise atransistor driver or an SCR circuit driver. The pulse outputs are thenapplied to the torque motor 42 in order to control the position of theantenna. One of the pulse outputs drives the motor 42 in a clockwisedirection while the other pulse outputs drive the motor 42 in acounterclockwise direction. If the antenna oscillation begins to slowdown, the pulse width of the outputs from the multivibrators isincreased to increased the drive to the antenna. Conversely, if thepeakleft and right positions of the antenna begins to exceed predeterminedposition limits, the width of the pulse outputs from the multivibrators184 and 186 decreases in order to decrease the driving motion to themotor 42.

tenna are fed through the ferrite circulator 52, previously described. Aload 200 is attached to the circulator 52 for use in testing of thereceiver. The transmitter magnetron is also connected to the circulator52. The circulator 62 is operated as a duplexer to permit the use of asingle antenna. The signals from the transmitter are sample through acoupler 202 and are fed through a power monitor circuit 204 to one inputof a NAND gate 206 and the built-in test equipment unit of the system.

The received signals from the antenna are transmitted through thecirculator 52 which may comprise the circulator R64l-LS manufactured andsold by Ferrotech, Inc. to a T-R switch 208. Switch 208 may comprise,for instance, the switch tube MA-3773 manufacture and sold by MicrowaveAssociates. A keep-alive circuit 210 biases the T-R switch 208 so thatthe switch tube is on the verge of breaking down in order to providesignal isolation in the known manner. The received signals are furtherfed through a ferrite switch 212 which provides additional attenuationfrom the transmit pulse. The ferrite switch may comprise, for instance,the LTW103 switch manufactured and solid by Forrotech, Inc. The PMTsignal is fed through a switch drive 214 in order to control theoperation of the ferrite switch 212.

The receiver signals are fed through a signal mixer comprising diodes216 and 218 connected at opposite polarity terminals. A resistance 220is connected across the diodes and is also connected to two inputs of aNAND gate 222. A solid state local oscillator 224 comprises a voltagecontrolled oscillator which supplies a 50 mw. signal at 1.458 gHz. to apower amplifier which increases the power to about one watt. The signalis then fed through a X 6 multiplier which preferably will comprise athin-film device. The resulting 250 mw. signal at 8.75 gI-Iz. is fedthrough 2. X4 multiplier to generate the final 10 mw. signal at 32.94gHz. :L-lSO mHz. This resulting signal is fed through an isolationcircuit 226 for mixing with the received signals. The frequency appliedto a linear amplifier 228 comprises a 60 mHz. intermediate frequency.

The IF signal is amplified in the linear amplifier 228 and a sensitivitytime control (STC) signal is applied to reduce the receiver saturationas the target range decreases. The STC signal is applied from agenerator 230 which is controlled by the PMT signal of the system. Theamplified signals are applied through a log post amplifier 232 wherein afinal 80 db gain is applied to the signal. A manual gain adjust controlis applied to the amplifier 232. Amplifier 232 has a linear-logarithmiccharaceteristic matched to the indicator dynamic range of the system toinsure the optimum display of radar target information. In the preferredembodiment, the amplifier 232 comprises seven amplification stages allof which operate for low power signals. The stages begin to saturate oneby one as the received signal becomes stronger, in order to give avoltage output which is a linear function of the input power to thesystem.

The output of amplifier 232 is applied to a video detector and amplifier234 which includes an emitter follower output. The detector 234regulates the 60 mHz. signal and operates as an envelope detector togenerate a video voltage pulse. A fast time constant (FTC) operation maybe applied at the amplifier 232 at the operators option. The FTCoperation serves to break up targets with large cross section, such asterminal buildings or residential areas, to prevent the saturation ofthe radar display. The video voltage pulses are applied to the indicatorof the system in a manner to be later described in greater detail.

A power monitor coupler 236 detects the signals transmitted from themagnetron and applies the signals to an AFC mixer comprising diodes 238and 240 connected at opposite polarity terminals. The output from theisolator circuit 226 is also monitored by a coupler 242 and applied tothe AFC mixer. The mixed signal, when the system is correctly operating,should be at the IF frequency and is applied through a limitingamplifier 244 to the AFC circuit 246. The output of the AFC circuit 246is applied to an input of the gate 222. The AFC mixer is 10 alsoconnected to two inputs of the gate 222. The AFC circuit 246 isconnected through a movable switch arm 248 which is movable betweenconnection to the local oscillator 224 and connection to a terminal 250.Terminal 250 is connected to a test load resistor 252 which is connectedto circuit ground.

The above-described AFC circuit maintains the receiver in frequencysynchronization with the transmitter. When the receiver is properlyoperating, the resultant signal from the AFC mixer should be at the IFfrequency. The mixed signal is applied to the AFC circuit 246 whichcontains discriminator and reference oscillator circuitry to generate aDC signal for control of the local oscillator 224. If it is impossibleto drive oscillator 224 to the desired frequency value, the resultingsignal is applied to the gate 222 and gate 206 in order to actuate afail indicator light 254 located on the aircraft instrument panel.Additionally, the presence of minimum transmitter peak output power issensed by the power monitor circuit 204, and the fail indicator light254 is energized if the output power falls below a predetermined minimumvalue.

The presence of mixer crystal bias current is also determined by thegate 222 and the fail indicator light 254 is illuminated if a currentfailure occurs. The illumination of the fail indicator light 254indicates a condition below design minimums, but does not necessarilyindicate a complete failure of the radar. Operator adjustment will berequired to judge when the information displayed by the system hasdeteriorated to a point of being completely unusable. To enable theoperator to make such a decision, the movable switch arm 248 may bemoved to the manual AFC test position and a movable switch arm 256 movedto connect the output of the linear amplifier 228 with a ringing line258.

The ringing line 258 generates a series of IF pulses spaced apredetermined delay apart, the pulses being fed through the amplifier232 for display upon the visual indication system of the invention. Sixhorizontal lines are displayed upon the radarscope, along with onevertical line at a position equivalent to the aircraft line of flight.This is accomplished by simulating an altitude input of 500 feet intothe display. The ringing line 258 is actuated by an attenuatedtransmitter pulse which excites the ringing line to create a pulse trainof 60 mHz. pulses which are 2000 feet apart in radar range and whichdecrease in amplitude with increasing range. The pulse trains producedby the ringing line 258 are displayed on the visual display ashorizontal lines to give an indication of correct operation of theantenna scan drive circuitry, the synchros and the display circuitry.

The amplitude of the injected pulses into the amplifier 232 is such thatthe six horizontal lines will be produced on the radarscope equivalentto two, four, eight, ten and twelve thousand feet ranges as viewed from500 feet. The seventh and subsequent pulses from the ringing line 258will fall below the system threshold and will thus not be displayed. Theline display provided by the ringing line gives a good indication ofproper receiver gain control setting, and if less than six lines aredisplayed when the system is put in the test position, the receiversensitivity is below operating minimums. The vertical strobe whichappears on the face of the visual display in the test mode is positionedby the antenna position synchro. The position of the vertical strobehorizontally relative to the center line is a measure of the linearityand accuracy of the display sweeps in the bore site position.

For additional description of radar receiver operation, reference ismade to Introduction To Radar Systems, chapter 8, by Merrill I. Skolnik,1962, McGraw-Hill.

RADAR TRANSMITTER FIG. 11 illustrates a block diagram of the presentradar transmitter. Input power is supplied to the present system via the400 c.p.s. conventional aircraft supply and passed through a linefiltering system which filters out the high I 1 frequency components ofthe aircraft voltage supply system. Three-phase relay switch 282switches the 400 cycle c.p.s. signal into a plate transformer 284.Overload protector circuit 286 prevents overload of the system and amagnetron filament relay circuit 288 supplies current to the magnetronfilament. Current is supplied tothe filament of the thyratron of thecircuit via a transformer 230.

A three-phase bridge 292 supplies rectified voltage via the L-sectionfilter 294 to a charging choke 296. A bleeder circuit 298 providesdischarge protection to the filter 294 when the system is turned ofif.The charging choke 296 includes inductors which provide a high impedanceload for the system. The voltage is applied via an inverse and chargingdiode assembly 300 to the transmitter thyratron 302. A trigger module304 supplies trigger pulses to the grid of the thyratron 1302. Theoutput of the thyratron 302 is applied through a pulse forming network306 which pulses when the thyratron 302 fires. The pulses from thenetwork 306 are applied through a pulse transformer 308 which suppliespulses to the magnetron 310'. The modulator of the invention is oilfilled in order to eliminate voltage breakdown at high altitudes and tocomply with decompression tests required to meet FAA environmentalspecifications. While a number of Ka band magnetrons are commerciallyavailable, the preferred embodiment of this system utilizes a L-4564tunable tube manufactured and sold by Litton Industries which ismodified to deliver 80 kw. peak power. Power for the magnetron 310 isapplied through a filament transformer 312. The output of the magnetron310 is applied to the circulator 52.

The synchronizer 58 supplies the PMT signal for timing operations of thesystem in the manner previously described.

In operation of the radar transmitter, the synchronizer 58 supplies aPMT signal to the trigger module 304 which delays the PMT to provide thethyratron trigger signal. The thyratron 302 normally has a highimpedance and the pulse forming network 306, which preferably comprisesa chain of LC networks, is charged up to the plate voltage applied tothe thyratron 302. When the thyratron is fired, the pulse formingnetwork 306 discharges through the pulse transformer 308 which steps upthe voltage to provide a high voltage drive to the magnetron 310. A 40ns. pulse width signal is generated by the pulse forming network 306 forapplication to the magnetron 310. The inverse diode assembly 300supplies a current return path for the energy which is not transferredto the magnetron due to mismatch between the pulse forming network andthe pulse transformer during discharge. The modulator is protected bythe overload protector circuit 286 for overloads caused by excessiveline currents, magnetron arcing or magnetron open circuits. In theoccurrence of an overload, the three-phase power to the modulator highvoltage supply is removed.

For additional description of radar transmitter operation, reference ismade to Introduction To Radar Systems, chapter 6, by Merrill I. Skolnik,1962, McGraw-Hill.

SWEEP GENERATOR SYSTEM Referring to FIG. 12, a block diagram of thesweep generator circuitry is illustrated. Radar altitude and barometricaltitude indications from altitude sensing equipment aboard the aircraftare fed to the limiter filter 350 such that the present system does notoperate upon any altitudes less than 50 feet. Filter 350 also tends tosmooth the altitude signals to eliminate sensing of towers, trees andthe like. The filtered signals are then fed into three waveforms shapingnetworks 352, 354 and 3-56. Shaping network 352 will be laterillustrated in greater detail and shapes the input waveform to generatean output signal which varies in voltage magnitude with respect toaltitude in the manner illustrated.

The shaped voltage is applied to an R-C circuit comprising resistor 358and capacitor 360. A diode clamp circuit 362 controls the chargingoperation of the R-C circuit. The shaped voltage output from the network354 is applied to an R-C network comprising 364 and capacitor 366, thestorage operation thereof being controlled by a transistor clamp circuit368. Similarly, the shaped voltage output from network 356 is applied toan R-C network comprising resistor 370 and capacitor 372. A transistorclamp 374 is connected across capacitor 372.

Each of the voltage outputs from networks 352, 354 and 356 are fed intoan input of a summing and subtracting circuit 376. The sum of the threeshaped voltages are therein subtracted from a reference input voltagesupplied via terminal 378 which is representative of a maximum elevationangle reference to the horizontal of 12.1". The output of the circuit376 is thus representative of the voltage required to bring the total ofnetworks 352-356 up to a maximum elevation angle of 12.1 The outputsignal of circuit 376 is fed to an R-C network comprising resistor 380and capacitor 382. A diode clamp circuit 384 is con nected acrosscapacitor 382 for control thereof.

An important aspect of the invention is the fact that the time constantsof the various R-C circuits of the sweep generator are varied. Thus, thevalue of capacitor 372 is times the value of capacitor 360, whilecapacitor 366 is 25 times the value of capacitor 360. The value ofcapacitor 382 is 5 times the value of capacitor 360.

The exponential voltage waveforms generated on capacitors 360, 366, 372and 382 (when the clamps are disabled) are each supplied to an input ofa summing amplifier 390, the output of which is applied through a selectrelay control 392 to a driver amplifier 394. The sum of the exponentialvoltages generated when the R-C networks are unclamped, and allowed tocharge to the voltage presented by its respective shaping circuit, willapproximately vary in accordance with a ratio of aircraft altitude andtime, or radar'range. The output of the driver amplifier 394 is utilizedto control the vertical line driver of the display radarscope. In themanner to be more fully described, this drive is a nonlinear signal toprovide a real-world perspective display of the approaching runway. Thelimited and filtered altitude signal is also applied to a voltage controlled delay circuit 398. The output of delay 398 controls the time atwhich a sweep width one shot multivibrator 400 is triggered, the outputof which is applied to control each of the clamp circuits 362, 368, 374and 384.

The PMT signal is also applied to the delay 398 and also to thesymbology, sky and range mark generator 402. The horizontal sweep signalis applied to the generator 402 for control thereof. After the delaydetermined by 398, the sweep width one shot 400 operates to unclamp theclamp circuits 362, 368, 374 and 384 such that the R-C storage networksbegin to charge according to their various time constants to the voltagelevels presented by their appropriate shaping network. The sum of thevoltage waveforms generated by the R-C networks, are representative ofthe ratio of the aircraft altitude and radar range. The delay induced bydelay circuit 398 is linearly proportional to aircraft altitude. At theend of the sweep width one shot period, the clamp circuits clamp thecapacitors to ground potential.

Upon unclamping of the various clamp circuits, the voltages generated onthe R-C networks are applied to the summing amplifier 390 for operationof the vertical line driver of the visual display. The modulated antennaposition signal supplied from the antenna sensing synchro is fed to anantenna demodulator circuit 406 which supplies a signal indicative ofthe antenna position to a cosine generator 408 and also to an amplifier410. A signal from a heading demodulator circuit 412 is also applied asan input to amplifier 410. The heading demodulator receives inputs fromthe control box, previously described, and from the heading gyro of theaircraft to generate a signal representative of the headingstabilization.

The output amplifier 4-10 is applied through a select relay 414. If theB-sweep or real-world sweep is selected at the relay 414, the outputfrom the amplifier 410 is fed to a line driver circuit 416 which is fedthrough a coaxial cable to the radarscope for driving the horizontalsweep for the B or for the real-world perspective modes. 'If the PPIsweep mode is selected at the relay 414, an output from the cosine 6generator 408 representing (or antenna position) is applied through anintegrating amplifier 418 and through an amplifier 420 for control ofthe horizontal line driver circuit 416.

The output from the heading demodulator 412 is fed to the amplifier 410for modification of the horizontal scan voltage. The visual display isthen compensated so that the runway remains in essentially the same spotduring small variations of the aircraft heading to prevent the runwayfrom moving during a rough approach.

Another output of the generator 408 representing cos 0 is appliedthrough a PPI or B'swee select circuit 424, the output of which is fedthrough an integrating amplifier 426 and an amplifier 428 to the selectrelays 392. The amplitude of the vertical B sweeps may be adjusted by avariation of the rheostat 430 at the B sweep circuit 424. The output ofthe amplifier 428 is applied through the select relay 392, if the Bsweep or PPI mode is Selected, to operate the vertical line drivercircuit 394 for control of the vertical sweep of the radarscope.

A relay select logic network 436 may be operated according to any offour range signals to operate a PPI and B sweep delay circuit 438. Inthe automatic range mode of operation, an 83 second sweep circuit isutilized in relay circuit 438. This sweep varies the PPI or B delay timeat a rate corresponding to the rate at which the aircraft would normallyapproach the runway during landing. Circuit 438 operates a PPI or Bsweep width circuit 440 which applies the signal to the integrator resetcircuit 442 for resetting of the integrating amplifier 426. The PMTsignal is applied to the S input of a flip-flop circuit 446 and thesweep width circuit 440 is connected to the R terminal of the flip-flopcircuit 446.

A plurality of inputs are applied to an intensity compensation circuit450 which controls a line driver 452 for control of the unblankingoperation of the visual indicator. The PMT signal is applied to theintensity compensation circuit 450, as are inputs from the sweep widthcircuits to determine the unblanking time and the derivative of thereal-world vertical sweep. The derivative of the real-World verticalsweep is added to a pulse during the'time that it is desired that thevisual indicator be unblanlced. The amplitude of the derivative of therealworld vertical sweep varies with altitude and time, such that thelow altitude signals are provided with greater intensity at the start ofthe sweep than are those for high altitude.

A roll demodulator circuit 454 senses the movement of the aircraft aboutthe roll axis from the aircraft gyro. The output of the demodulator 454is fed to a four quadrant multiplier 456 which generates an input forthe summing amplifier 390. The real-world horizontal sweep is also fedto the multiplier 456 for multiplication thereof by the roll angle andfor addition to the vertical scan signal. Thus, the circuitry senses therotation of the aircraft and correspondingly rotates the picture of therun way displayed upon the radarscope to assist in the realworlddisplay.

In operation of the system in the B sweep mode, the select relays 392and 414 are moved to the B sweep mode position with the use of the modeselect switch on the aircraft display instrument panel. The modulatedantenna position is thus fed through the antenna demodulator circuit 406and through amplifier 410 for operation of the horizontal line driver416. The horizontal sweep of the visual indicator is thus proportionalto the antenna sweep. The vertical sweep width of the B sweep mode isset at circuit 440 and the integrating amplifier 426 and the amplifier428 operate the vertical line driver 394 to provide a linear rampincreasing as a function of time to operate the vertical sweep of thevisual indicator. The sweep ends at whatever sweep width is set into thecircuit 440. The resulting sweep thus provides a conventional B sweepdisplay on the radarscope.

If a delayed B sweep mode of operation is chosen at the selected relays,the display of a segment of a detected area is provided on theradarscope display. The circuit operates as previously described, withthe exception that the delay circuit 438 operates to prevent operationof the vertical B sweep for the selected time interval chosen at therelay select logic 436.

The automatic range selection may be made at the relay select logic 436to thereby switch in the 83 second sweep and the delay circuit 438. Thismode of operation varies the delay time at which the vertical sweep isinitiated and thus compensates for the speed of the aircraft, so'thatthe detected airport runway maintains at essentially the same spot onthe radarscope, thus easing the problems of identifying and recognizingthe position of the runway to the radarscope. The runway becomes largeras the aircraft approaches, but remains in view on the radarscope toeliminate switching of ranges into the scope display.

In the PPI operation mode, the select relays 392 are switched to the PPIposition. The vertical line driver 394 is thus driven by a positive rampsignal from 408 whose amplitude is proportional to the cosine of theantenna signal fed into the antenna demodulator circuit 406. Thehorizontal line driver 416 is driven through the cosine 0 generator 408,the integrator amplifier 418 and amplifier 420 according to anapproximation of range multiplied by the sine of the azimuth angle,which for the present system may be approximated as range multiplied bythe azimuth angle. As previously noted, both the B sweep, delayed Bsweep and PPI displays are conventional and will not be discussed infurther detail.

In the operation of the real-world display of the invention, the selectswitches are suitably positioned and radar or barometric altitudesignals are fed into the 50 feet limiter filter 350. The filtered andlimited signal is fed simultaneously into the waveshaping networks 352,354, and 356, wherein the voltages are shaped according to altitude inthe illustrated manner. The shaped voltages are applied respectively oncapacitors 360, 366 and 372 and clamp circuits through resistors 358,364, 370. Additionally, a voltage representative of the differencebetween a l2.1 elevation angle and the sum of the outputs from thewaveshaping networks is applied to the R-C network defined by resistor380, capacitor 382 and the diode clamp. Upon unclamping of the variousclamp circuits, the R-C networks begin charging up according to theirrespective time constants. The summing amplifier 390 thus generates avertical drive which is a nonlinear function which is approximated by:

B=h/r (1) wherein fl=the elevation angle, h =the aircraft altitude, andr=the instantaneous target range of the radar signals.

Driving the vertical sweep of the radarscope according to the nonlinearEquation 1 operates to provide an airport runway display on theradarscope which directly corresponds to the real-world perspective viewof the runway from the aircraft. In other words, the approaching runwaywill be displayed with a straight forward runway edge having straightrunway sides converging toward the rear runway edge according toreal-world linear perspective. Such a display not only assists the pilotin accurately determining the lateral offset of the aircraft from therunway and the area of touchdown on the runway, but the display assiststhe pilot in making a smooth transition from the instrument runwaydisplay to the actual view of the runway through the cockpit windowduring landing.

